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The present invention relates to flat ribbon cables and more specifically to a flat ribbon cable having a controlled impedance that is suitable for applications involving high speed data transmission.
The use of flat ribbon cables throughout the electronics industry is wide spread. Such cables are often desirable because they are flexible and are generally easy to employ from the perspective of cable management. Additionally, ribbon cables may be easily terminated using insulation displacement connectors (IDC) as is known in the art. Typically, ribbon cables are formed by extruding an insulating material, such as plastic, around a plurality of conductive wires that are maintained in parallel co-planar relation.
It has been observed however, that traditional flat ribbon cables are not suitable in some high speed applications, such as those encountered with the Universal Serial Bus 2 (USB2) which can accommodate a data rate of 480 Mbps. More specifically, it has been difficult to produce a ribbon cable that is suitable for use at such high speeds using traditional fabrication techniques and structures due to the inability to accurately control the impedance of the signal carrying conductors along the length of the cable.
Accordingly, it would be desirable to have a flat ribbon cable which would present a controlled impedance along the length of the cable and would be suitable in high speed data applications, such as those encountered in USB2 applications. It would be further desirable if such a ribbon cable could be terminated using conventional insulation displacement connectors to facilitate rapid and low cost manufacture of terminated cables.
In accordance with the present invention, a flat ribbon cable is disclosed that has a controlled impedance along the length of the cable for at least some of the conductors contained within the cable.
The disclosed flat ribbon cable includes a plurality of parallel and generally co-planar conductors. Each of the conductors has first end portions at respective ends of the flat ribbon cable. The first end portions each have a generally circular cross section. Each of the conductors has a central portion that has a generally rectangular cross section such that the width of the cross-section is greater than the height. A second portion at each end of the cable comprises a transition portion that is disposed between the respective first end portion and the central portion of the conductor.
A generally planar conductive shield layer is laminated over some or all of the conductors of the ribbon cable. The spacing between the shield layer and the respective adjacent conductors is accurately maintained to provide a controlled impedance for the conductors along the length of the cable. At least one drain wire is provided that extends along the length of the cable and is conductively coupled to the shield layer to allow the shield layer to be conductively coupled to ground or otherwise terminated at a low impedance point within an electrical circuit. In one embodiment the drain wire is generally coextensive in length with the ribbon cable conductors and is disposed directly above one of the conductors and between the shield layer and the insulating material. Since the end of the drain wire is disposed directly above one of the conductors, the respective conductor and the drain wire may be captured with a single contact of an insulation displacement connector (IDC) so as to conductively couple the respective conductor to the shield layer via the drain wire. One or more drain wires may be employed. Multiple shield layers may be provided with at least one drain wire conductively coupled to each of the shield layers. Each of the shield layers may be conductively coupled to a conductor of the cable by conductively coupling the respective drain wires to a corresponding conductor within the ribbon cable.
The presently disclosed flat ribbon cable may be produced by laminating the plurality of conductors between two insulating layers or via an extrusion process to permit high speed manufacture of cables of any desired length. Additionally, the ribbon cable may be formed by a lamination process and the shield may be encased via a secondary extrusion process.
Other features, advantages and aspects of the presently disclosed invention will be apparent from the detailed description of the invention that follows.